Nature has created many of the skeletal joints of the human anatomy such that the movement of one bone relative to the other bone, or bones, at the joint is best described as a rotational motion within a definable envelope. Specifically, the normal movement of a bone at a joint is a rotation of the bone about two different axes which are non-perpendicular and non-intersecting. Several mechanical advantages result from this fact. First, the kinematics of anatomical joints allows for more than just simple rotation in a plane. Specifically, because the two axes of rotation are non-perpendicular and non-intersecting, rotation of the bone about these two axes manifests rotations in all three anatomical planes (sagittal, transverse and coronal). Equally important, because only two axes of rotation are involved, anatomical joints require fewer muscles to achieve this rotation and maintain stability of a bone about the joint than would be required for a structure which uses three axes of rotation to achieve the same movement. Thus, anatomical joints are very mechanically efficient. The result of all this is what people perceive as normal skeletal movement.
Many prosthetic replacements for diseased or damaged joints have been developed and used over the years. Clearly, the intention in developing these joints has been to effectively duplicate the natural kinematics of the particular anatomical joint that is to be replaced. To do this, however, there has been no successful attempt to duplicate or mimic the natural boney structure of the joint. Instead, prosthetic joint designs have typically been driven by conventional considerations of joint movement relative to the anatomical reference planes. This has caused many to overlook the fact that normal bone movement about a joint can involve simultaneous interrelated movements in all of the reference planes. As a consequence of the conventional approaches used to design prosthetic replacement joints, the joints have been designed with either a single axis of rotation or with orthogonal axes of rotation. For example, U.S. Pat. No. 4,944,758, which issued to Bekki et al. for an invention entitled "Artificial Finger Joint", discloses a single axis joint which guides one member of the joint as it bends relative to the other joint member in a prescribed direction. As another example of conventionally designed prosthetic replacement joints, U.S. Pat. No. 4,229,841, which issued to Youm et al. for an invention entitled "Wrist Prosthesis", discloses a joint which provides for relative movement of the components about two substantially perpendicular axes. As an example of a three axes prosthesis, U.S. Pat. No. 4,349,922, which issued to Agee for and invention entitled "Joint Prosthesis with Improved Articulation Means", discloses a combination of linking pins which allows for members of the joint to pivot about three different axes. Unlike the above cited references which include interconnected members, U.S. Pat. No. 4,959,071, which issued to Brown et al. for an invention entitled "Partially Stabilized Knee Prosthesis", discloses a prosthetic joint which includes members that are not interconnected, but which rotatably slide relative to each other. Still, despite this distinction, the prosthetic joint disclosed by Brown et al. is restricted to rotation about a single axis.
None of the above cited references either teach or suggest designing a prosthetic replacement joint by first analyzing the kinematics of the joint itself. If this were done, the naturally occurring two non-perpendicular and non-intersecting axes of rotation, which are characteristic of a normal joint, would become evident. With this in mind, the present invention has recognized that the abutting surfaces of normal bones at a joint are surfaces of revolution. Specifically, each of the abutting surfaces of the bones at a joint is a surface of revolution about the joint's two non-perpendicular and non-intersecting anatomical axes of rotation. Further, the present invention recognizes that this surface of revolution is a torus, and specifically a skewed torus, which has parameters that can be geometrically varied to recreate the particular bone joint surface of interest. Using these observations, the present invention recognizes that the boney cartilaginous structures of an anatomical joint can be reproduced for a prosthesis and employed to restore normal joint kinematics.
It is interesting to note that the anatomical shapes of abutting bone surfaces at a joint have developed naturally to take advantage of a bone's capacity for carrying relatively large compressive forces. At the same time, these bone surface configurations minimize the transmission of torsional and shear forces which are not as well tolerated by boney structures as are compressive forces. As implied above, the naturally occurring configurations of abutting bone surfaces which achieve this force distribution can be characterized as surfaces of revolution about two non-perpendicular and non-intersecting axes.
The carpometacarpal (CMC) joint of the thumb is a case in point. Anatomically, the CMC joint establishes a rotational movement of the first metacarpal bone relative to the trapezium bone which is properly characterized as a mutual rotation about two off-set axes. It happens that one of these axes is located on the trapezium while the other is located on the first metacarpal bone. Interestingly, nature has provided the muscle network in the thumb which must effectively moves the respective bones at the CMC joint in rotation about these axes. Specifically, but without going into great anatomical detail, nature provides a pair of muscles for each off-set axis of the CMC joint. The individual muscles in each of these pair of muscles act opposite to each other to move the bones at the joint in rotation about the particular off-set axis with which they are associated. Thus, the complete natural movement of the thumb at the CMC joint is accomplished by only four muscles, which act in pairs. Further, nature uses these same muscles to move the other joints of the thumb, namely the metacarpal phalangeal (MP) and interphalangeal (IP) joints, which are distal to the CMC joint.
Prior attempts to create an effective replacement prosthesis for the CMC joint have typically relied on a ball and socket structure. These prosthesis moved about three perpendicular intersecting axes of rotation, and the placement of the axes of rotation relative to the thumb muscles and the external loads was not considered in their design. As indicated above, the thumb has only four muscles associated with the CMC joint. On the other hand, a ball and socket configuration would require six muscles, or three pairs, to effectively articulate the thumb about the axes of the joint. As we know, however, nature does not provide the needed extra pair of muscles which would be required for a three axis joint. Consequently, if a three axis joint is used, the need for additional muscle action upsets the balance of the entire thumb. Unfortunately, due to the interlinking involved, this imbalance is extended to include an adverse effect on operation of the MP and IP joints as well as the CMC joint.
An additional difficulty encountered in using a ball and socket type structure, as a CMC joint prosthesis is the inherent requirement for generous resection of the bones. Typically, straight cuts or hollowing out of the bone is necessary before such prosthesis can be attached to the trapezium and metacarpal bones of a CMC joint. Procedures using previously known CMC prosthesis joints have required removal of the trapezial joint surface, and in some instance the whole bone has been removed. Obviously, excessive resection or removal of bone is to be avoided.
The present invention recognizes that a prosthetic CMC joint can be manufactured and used which will effectively duplicate the anatomical structure of the joint. Further, the present invention recognizes that bone can be conserved by following a strategy which relies on resurfacing arthroplasty with limited, if any, bone resection requirements.
In light of the above it is an object of the present invention to provide a method for modeling a prosthetic joint which produces a prosthetic replacement joint which will restore normal joint kinematics. Another object of the present invention is to provide a method for modeling a prosthetic joint which produces a prosthetic replacement joint that maintains the mechanical advantage of the muscles which cross the joint. Still another object of the present invention is to provide a method for modeling a prosthetic joint which produces a prosthetic replacement joint which recreates the normal joint's axes of rotation. Yet another object of the present invention is to provide a method which can be employed for modeling different types of prosthetic replacement joints. Still another object of the present invention is to provide a prosthetic carpometacarpal joint that allows the stresses which act across the prosthesis to be dissipated over a large area of bone to increase the load bearing capability of the prosthesis and reduce the likelihood of failure of the prosthetic joint. It is another object of the present invention to provide a prosthetic carpometacarpal joint that allows for bone conservation by relying on bone surface replacement, rather than extensive bone resection or removal, in preparation for prosthesis attachment. Another object of the present invention is to provide a method for modeling prosthetic replacement joint which is relatively easy to accomplish and comparatively cost effective.